Quantum well lasers have very favorable properties for use in optoelectronic integrated circuits and optical communication systems. In this regard, there has been extensive study of quantum well lasers, including single quantum well lasers, fabricated from heteroepitaxial structures comprising contiguous layers of AlGaAs and GaAs. The principles of operation of such devices are described, for example, in U.S. Pat. No. 3,982,207 issued on Sept. 21, 1976 to Raymond Dingle and Charles H. Henry, hereafter "Dingle." In such devices, the quantum well is a thin layer, here called an active layer, comprising material of relatively low bandgap, bounded on both sides by layers, here called confinement layers, comprising material of higher bandgap. The active layer is thin enough for the optical emission from the recombination of electrons and holes in the layer to exhibit a quantum mechanical size effect. The active layer is usually less than about 300 .ANG. thick.
Typically, the low bandgap material of a quantum well laser is GaAs, and the higher bandgap material is AlGaAs. Electrons and holes created by optical or electrical pumping are captured in the quantum well, where they desirably recombine radiatively, thus emitting radiation. It is advantageous to confine the laser radiation in thin-film optical waveguides adjacent to the active layer. Thus, in a so-called "separate confinement heterostructure" laser, at least one confinement layer also functions as a waveguide. This is achieved by bounding the waveguiding confinement layer on the side opposite the active layer by a layer having a lower refractive index. Although the active layer has a higher refractive index than the confinement layers, it is too thin to confine radiation and act as a waveguide. Thus, the recombining carriers are confined in the active layer, and the radiation is "separately confined" in one or more waveguiding layers.
The (Al, Ga)(As) material system is advantageous for growing heteroepitaxial structures because the mole fraction of aluminum, relative to gallium, can be varied from 0 to 100% without substantially affecting the lattice constant. As a result, heteroepitaxial structures can be grown without appreciable lattice strain. However, the laser emission from this material system occurs at wavelengths that are too short to be useful for many applications in optical communications. That is, because of considerations pertaining to transmission loss in optical fibers, the most useful wavelengths for optical communication are 1.3 and 1.55 .mu.m. However, the band gaps of (Al, Ga)(As) materials are too large for laser emission to take place at such wavelengths. A typical wavelength of laser emission from GaAs, for example, is about 0.87 .mu.m.
As a consequence, there is great interest in alternative, lower bandgap III-V material systems in the (In, Ga)(As, P) system. Quantum well lasers of the (In, Ga)(As, P) material system are advantageous because they can be operated at the wavelengths most useful for optical communication; namely, at 1.3 and 1.55 .mu.m. The quaternary material InGaAsP is advantageously used for the confinement regions, and the ternary material InGaAs is advantageously used for the active regions. The binary material InP has a smaller refractive index than the quaternary material, and therefore quaternary layers bounded by InP are readily used as waveguiding layers.
However, when the lasers are grown using conventional metalorganic vapor-phase epitaxy, it is found that the quaternary material does not grow well on the ternary material. In particularly, the top quaternary layer is found to exhibit an unacceptable amount of strain and an unacceptably high density of dislocations. Dislocations are undesirable because they enhance the rate of nonradiative recombination, and thus decrease the efficiency of the laser. In consequence, the threshold current of the laser is increased.